docs: def _normalize_url(graph: BELGraph, keyword: str) -> Optional[str]:  # FIXME move to utilities and unit test
      """Normalize a URL for the BEL graph."""
      if keyword == BEL_DEFAULT_NAMESPACE and BEL_DEFAULT_NAMESPACE not in graph.namespace_url:
          return BEL_DEFAULT_NAMESPACE_URL
  
      return graph.namespace_url.get(keyword)